1. Field of the Invention
The present invention relates to a technique for improving a yield of aluminum sulfate in a preparation of aluminum sulfate. In addition, the present invention relates to an industrial composition comprising aluminum sulfate, which can be advantageously used as a wastewater treatment agent, a paper sizing agent, a tanning agent for hides, a clarifying agent for fats and oils, or a catalyst for industrial use.
2. Description of Related Art
Aluminum sulfate is an industrial chemical widely used as a treatment agent (inorganic flocculant) for sewerage, wastewater from plants and wastewater from water supply, a paper sizing agent, a tanning agent for hides, a clarifying agent for fats and oils, or a variety of catalysts.
Aluminum sulfate is generally produced by a method in which sulfuric acid is charged into a reaction vessel made of heat resistant, acid resistant materials, aluminum hydroxide is added to the sulfuric acid and heated, then the resultant aluminum sulfate solution having a high concentration is diluted with water, stored in the reaction vessel and cooled, followed by adjustment of the concentration, thus obtaining aluminum sulfate as a product (see Unexamined Japanese Patent Application Laid-Open Specification No. 54-106098 and Examined Japanese Patent Application Publication No. 7-25541).
However, the above conventional method for producing aluminum sulfate requires heating of the reaction system to about 100 to 130° C. and maturing of the reaction mixture for several tens minutes to about 2 hours. Further, for promoting the reaction, sulfuric acid of reagent grade having a high concentration (90 to 98% by weight) is inevitably used. Generally, waste sulfuric acid from semiconductor plants has a sulfuric acid concentration of about 70 to 90% by weight. Therefore, when it is attempted to utilize waste sulfuric acid effectively, the development of a method which can produce aluminum sulfate efficiently from sulfuric acid having a lower concentration is desired.
On the other hand, when aluminum sulfate is used as the above-mentioned wastewater treatment agent, it has a problem that aluminum sulfate containing no other chemicals in a large amount cannot exhibit a satisfactory aggregation rate, as compared to other treatment agents, for example, polyaluminum chloride (PAC). Therefore, recently, PAC is frequently used instead of aluminum sulfate, but aluminum sulfate has an advantage in that waste sulfuric acid can be used as a raw material therefor. Thus, from the viewpoint of achieving effective utilization of waste materials (saving of materials), it has been strongly desired to develop aluminum sulfate having an aggregation performance comparable to or more excellent than that of PAC.